tui: export sessions using consistent Filesystem API instead of Bun.write

packages/opencode/src/cli/cmd/tui/routes/session/index.tsx

@@ -907,12 +907,12 @@ export function Session() {
             const filename = options.filename.trim()
             const filepath = path.join(exportDir, filename)
 
-            await Bun.write(filepath, transcript)
+            await Filesystem.write(filepath, transcript)
 
             // Open with EDITOR if available
             const result = await Editor.open({ value: transcript, renderer })
             if (result !== undefined) {
-              await Bun.write(filepath, result)
+              await Filesystem.write(filepath, result)
             }
 
             toast.show({ message: `Session exported to ${filename}`, variant: "success" })